Power line carrier systems are for transmitting and receiving various kinds of data, utilizing power lines as a transmission channel. There have been proposed a variety of power line carrier systems some of which have been already put to practical use. It is known that coupling of power lines to a modem in such systems is established by a coupling circuit composed of a capacitor, a coupling circuit composed of a transformer, or coupling circuit composed of a capacitor and transformer. For instance, in a known system, the primary winding of a transformer is connected to a power line through a capacitor whereas the secondary winding of the transformer is connected to a receiving circuit. Another known system includes a communication device that is coupled through a transformer to power lines.
There is known a transformer for use in communication, in which multi-core parallel lines formed by connecting a plurality of core wires in parallel are wound around a core to form a primary winding and an ordinary core wire is wound around the core to form a secondary winding. According to another known arrangement, three secondary windings connected in series are provided for a transformer, two of which are connected to a transmitting circuit through a resistance and a receiving circuit is connected to the three serial secondary windings through a resistance, thereby forming a secondary winding portion for common use in transmission and reception. With this arrangement, the level of transmitter signals is increased while a reverberation elimination circuit is simplified (Japanese Patent (Kokai) Publication No. 8-98277, Japanese Patent Publication Nos. 2001-186063, 2001-267139, 2001-136107).
Transformers are generally classified into two types, namely, for power line use and communication use. In power line transformers, power conversion efficiency susceptible to noise, distortion or the like is an important factor, whereas communication transformers give great importance to distortion-free transmission of minute signals. Transformers for use in a power-line carrier system are similar to communication transformers but need to be connected to power lines having a voltage as high as 100V or more, whereas communication transformers are connected to low-voltage communication lines. In addition, the fact that noise components having great amplitude are superimposed on such power lines should be taken into account.
To sum up, the requirements for transformers for use in a power-line carrier system are as follows: First of all, they are required to have a high current drive capability, small size, high cost performance and high efficiency. Secondly, their line capacity should be as small as possible in order to attain the highest possible common mode impedance. Thirdly, it is necessary to make ground balance as large as possible at the transmitting end to minimize a leak magnetic field by minimizing a common mode leak current. At the receiving end, the possible greatest ground balance is also required for improved common mode noise resistance. Fourthly, it is necessary to make transmission THD (Total-Harmonic-Distortion) as great as possible in order to minimize undesired out-of-band spurious response and to make noise THD as great as possible in order to enable reception of signals of minute amplitude in environments where great amplitude noise exists. Fifthly, the current peak point needs to be within the transmission band in order to enable efficient high current drive and the lowest possible impedance drive. Sixthly, it is necessary to make transmission characteristics as flat as possible in order to achieve stable receiving characteristics.
Concretely, the above requirements are met by setting the following conditions. At the transmitting end, the drive current at the line side is 1.4 A or more in maximum, and a low drive impedance within the range of 1.0 to 3.0Ω is obtained. For effective input of a signal, the current peak point is set in the transmission band (150 kHz to 450 kHz). The distortion of a transmitter signal (transmission THD) is adjusted to the spurious modulation standards (56 dBμV or less at 450 kHz to 5 MHz, and 60 dBμV or less at 5 MHz to 30 MHz) stipulated by the rule No. 46 of the Radio Law. At the receiving end, the ground balance is 50 dB or more, the noise THD is 60 dB or more, the group delay characteristic is 3 μs or less within the transmission band, and the fine receiver signal level is −95.0 dBm/kHz or less (when the required S/N is 15 dB).
The conventional transformers called “communication transformers” and the conventional transformers for a power line carrier do not satisfy all of the six requirements listed above nor propose any means for overcoming the above problems.
Therefore, a primary object of the invention is to provide a modem coupling circuit for a power line carrier, the circuit including a transformer which is designed to satisfy all of the first to sixth requirements.